1. Field of the Invention
This invention relates to rare earth metal stabilized-aluminum deficient crystalline alumino-silicate zeolites, and methods for their preparation.
2. Description of Information Disclosures
Crystalline aluminosilicate zeolites having a high silica to alumina mole ratio may be prepared by various methods. High silica to alumina mole ratio zeolites often have desirable characteristics for use in certain processes. With some types of zeolites, it is possible to control silica to alumina mole ratio during the synthesis step. However, this does not work well for zeolites having the same structure as natural faujasite. An example of such a zeolite is zeolite Y. With these types of zeolite, it is usually necessary to do some post-synthesis step in order to modify significantly the silica to alumina mole ratio. One way to do this is to add silica to the structure. A general method of doing this involves exchange of alumina in the crystal structure (i.e., framework) of the zeolite for silica from an outside source. This can be accomplished by treatment with ammonium-fluorosilicate as described in U.S. Pat. No. 4,503,023. A second method uses silicontetrachloride as the source of silica (H. K. Beyer and I. Belenykaja, Catalysis by Zeolites, page 203, 1980, Elsevier Scientific Publishing Company, Amsterdam). The older and more typical way of increasing the silica to alumina ratio of the Y zeolites involves some sort of dealumination. The literature on these methods is often confusing as the term "dealumination" or "alumina deficient" does not always have the same meaning. Several different zeolites which are quite different but that have all been described as "dealuminated" or "alumina deficient" are described in more detail below:
1. In Case 1, the terms are used to describe the removal of alumina from the zeolite structure by converting it from the tetrahedral form required by the zeolite structure to an octahedral form. This octahedral or non-zeolite alumina is not physically removed so the silica to alumina ratio as measured by chemical analysis has not changed but the silica-alumina ratio of the remaining zeolite has increased. An example of this sort of dealumination is the transformation of Y to an ultrastable Y zeolite (USY). The silica to alumina mole ratio of the initial Y zeolite is usually near 4 whether measured by elemental analysis or some method that measures the actual ratio in the zeolite crystal structure. After hydrothermal treatment, the silica to alumina mole ratio in the zeolitic crystal structure is typically about 8, but the silica to alumina ratio as measured by elemental analysis has remained unchanged. The determination of the silica to alumina mole ratio in the zeolitic crystal structure is usually done by X-Ray measurements of the unit cell size or by solid state nuclear magnetic resonance measurements.
2. In a second case, the term "dealumination" is used to describe the removal of the octahedral of non-zeolitic alumina such as produced in Case 1 above further without removal of tetrahedral or zeolitic alumina. For example, this can be done by chemically treating a sample of zeolite that has already been transformed from Y to ultrastable Y zeolite. The chemical treatment is kept mild enough so that only the alumina that is not part of the crystal structure of the zeolite will be removed. In this case, the silica to alumina ratio as measured by chemical analysis will increase while the silica to alumina ratio of the remaining zeolite as measured by X-Ray diffraction or nuclear magnetic resonance will not increase.
3. In Case 3, the term "dealumination" is both less precise and more complicated because it describes situations where alumina is removed both from the zeolitic crystal structure and at the same time physically removing it from the sample of zeolite that is being treated. Examples of ways of carrying this out usually involve either some form of acid treatment or use of a complexing agent such as ethylenediaminetetraacedic acid (EDTA). In this form of dealumination, there is often a reordering of the zeolite crystal structure. It is the degree to which this reordering takes place that distinguishes several subcases of this type of dealumination. This reordering of the zeolite crystal structure is usually described as taking place by the migration of silicon atoms from some other part of the zeolite to fill the defect sites or hydroxyl "nests" left in the structure by the removal of the aluminum atoms. In some cases, this reordering of the zeolite crystal structure is deliberately encouraged by a post-treatment with either heat or steam.
a. The most common subcase is that as the alumina is removed from the zeolite crystal structure, the reordering takes place so that the silica-alumina ratio in the crystal structure has been increased by the same amount as the alumina that is chemically removed.
b. In this subcase, the conditions are carefully controlled in order to remove aluminum from the zeolite crystal structure so that little or no reordering of the crystal takes place. In this case, the empty sites or hydroxyl nests created by the removal of aluminum atoms still remain. If no reordering has taken place and the starting zeolite had no extraneous non-zeolitic alumina present, the silica to alumina ratio as measured by the unit cell size will not have changed but the silica to alumina ratio as measured by chemical analysis will have increased. It is this type of alumina deficient products that are the subject of this invention.
c. The third subcase occurs frequently but may not always be recognized. If the conditions of an acid treatment to remove alumina from the structure are too severe, the zeolite structure suffers partial collapse so that the remaining crystal structure has a higher silica-alumina ratio as measured by magnetic nuclear resonance or unit cell size than would be measured by chemical analysis. This partial collapse is very similar to the transformation of zeolite Y to an ultrastable Y zeolite.
There are several accepted techniques for measuring the silica to alumina ratio in the zeolite crystal structure in the presence of non-zeolitic silica or alumina. One of the newest methods is solid state nuclear magnetic resonance which can distinguish between aluminum atoms that are in a tetrahedral coordination, which is required by the Y zeolite structure, or in an octrahedral configuration which is typical of the alumina that is still present but no longer part of the crystal structure of the zeolite. Another and slightly less common method is the use of infrared spectroscopy as described by Maxwell, et al. (Maxwell, I. E.; Van Earp, W. A.; Hayes, G. R.; Couperus T.; Huis R.; and Clague, A. D. H., Journal of The Chemical Society, Chemical Communications, 1982 p. 523). A paper by Engelhard, et al. gives a good description of the NMR and infrared techniques and compares the two methods relative to chemical analysis (G. Engelhard, U. Lohse, V. Patzelovga, M. Magi, and E. Lippmaa, Zeolites, Vol. 3, page 233, 1983). A third and older technique is the use of X-Ray diffraction to measure the unit cell size of the zeolite crystal. The unit cell size method depends on the fact that a silicon-oxygen bond is shorter than that of an aluminum-oxygen bond so that as alumina is removed from the crystal structure and replaced by silica, there is an overall contraction of the unit cell size which is directly proportional to the silica to alumina ratio in the zeolite crystal structure. This technique is described in U.S. Pat. No. 3,056,400 by Eberly, et al. and in the work of Breck and Flannigan and in the Kerr U.S. Pat No. 3,442,795. The Kerr reference is particularly useful because, in addition to pointing out the existence of hydroxyl "nests", it discusses, in columns 15 and 16, the fact that aluminum atoms can be removed from the tetrahedral sites of the zeolite without the replacement by silicon atoms. This is an example of one of the types of dealumination described earlier (subcase 3b) and it is in this sense that the term "dealuminated zeolite" is used to describe the products of this invention.
It is well known in the art that exchange of Y type zeolites with rare earth metal ions increases both their thermal and hydrothermal stability. The exact mechanism of this stabilization is still open to speculation. The two most common theories cited in the literature are as follows: (1) the rare earth metal ions, by virtue of occupying exchange sites, slow down the rate that aluminum atoms leave the zeolite crystal structure; (2) the physical presence of the rare earth metal ions in the sodalite cage act to support physically the sodalite structure which is a key building block of the zeolite Y. Some evidence of the second mechanism is given in a paper by Sherzer, et al. (Journal of Physical Chemistry, Vol. 79, p. 1194, 1975). However, there is no suggestion in the prior art that exchange by rare earth metal ions can stabilize a faujasite type structure that contains a large number of hydroxyl nests that have not been filled by silicon atoms. The present invention is based on the finding that this particular form of alumina deficient zeolite can be stabilized in this dealuminated state. The use of rare earths to stabilize a Y or USY zeolite so that it retains this alumina deficient structure in the presence of heat or steam has not been reported in the prior art.
U.S. Pat. No. 3,442,795 discloses dealuminating a sodium Y zeolite with ethylenediaminetetracedic acid complexing agent and then ion exchanging the dealuminized zeolite with the rare earth metal-containing solution. Most of the examples in this patent deal with dealumination of the Y zeolites with ethylenediaminetetracedic acid. The data given in Table E show an example of dealumination where the aluminum is removed from the crystal structure and physically from the zeolite without the reordering and the hydroxyl "nests" being filled by silica atoms. This is shown by Examples 5, 6 and 7 where the silica to alumina mole ratio as measured by chemical analysis increases from 5.8 to 9.12, while the unit cell size or lattice constant remains essentially the same. This is also shown in Examples 9 through 12 where a low silica to alumina sodium Y zeolite was given a similar treatment. These two sets of examples illustrate two other points that are also shown in the present invention. First, the degree of dealumination that can be accomplished without loss of crystallinity is dependent upon the silica to alumina ratio of the starting material. A second point illustrated by these data is that if dealumination is carried too far, in addition to a loss in crystallinity, there will be a spontaneous lowering of the unit cell size even under mild conditions. This is shown by comparison of Example 8 with Example 7 and Example 13 with Example 12. Examples 20 and 21 of this patent give examples of dealumination followed by rare earth exchange, followed by a subsequent steaming. The final steaming conditions in these examples are so severe that the zeolite crystal structure could no longer be alumina deficient. These materials would fit under Case 3a described above.
U.S. Pat. No. 3,506,400 discloses impregnating a dealuminized zeolite with metal cations. The term "dealuminized" as used in this patent involves the case where alumina is first removed from the crystal structure by a steam treatment and then the amorphous alumina that is still present in the sample is chemically removed by treatment with a chelating agent or by acid extraction as in Case 2 described earlier. In the examples given in Table 6 of this patent where unit cell size data are available on the faujasite type zeolites, there is no evidence that the procedures taught have resulted in a material that contains hydroxyl "nests" or that the zeolite crystal structures are alumina deficient. The products of this patent would not be expected to be deficient in zeolitic alumina. Eberly teaches a three step process that comprises a partial removal of soda so that a critical heat-steaming step will remove tetrahedral alumina from the zeolite crystal structure and convert it to octrahedral or amorphous alumina. When alumina is removed from the zeolite crystal structure by hydrothermal treatment, the remaining structure recrystallizes to a higher silica to alumina ratio. Hence, this new structure is believed to be complete so that it does not have any defect sites that are missing tetrahedral alumina atoms. The third step is an acid leaching to remove the amorphous alumina that was created by the hydrothermal treatment. (This is a Case 2 dealumination.)
U.S. Pat. No. 4,093,560 discloses dealuminizing an aluminum silicate zeolite, which may be zeolite Y, with a solution comprising an inorganic acid such as HCl and a salt of a complexing agent such as ethylenediamenetetracedic acid. The dealuminized zeolite is then ion exchanged with a solution of rare earth metals. This patent teaches a special method of leaching alumina from a zeolite that avoids the severe losses in zeolite stability that are normally associated with a highly alumina deficient structure. The teaching is to carry out the leaching at such a slow rate that silicon atoms can migrate to fill the empty sites or hydroxyl "nests" left in the structure by the leaving aluminum atoms (column 2, lines 45 to 49). This is analogous to the recrystallization of the zeolite to a defect free structure of a higher silica to alumina ratio achieved by hydrothermal treating such as was taught by Eberly. The whole purpose of this special method of leaching is to avoid a large formation of hydroxyl "nests" or defect sites which would make the material less thermally stable. Note that in Example 3 the mixture was stirred for two additional days after the 80 hour period during which acid had been added in the leaching step. This is to allow time for the silicon atoms to migrate to fill any empty sites left by the removal of aluminum atoms from the zeolite crystal structure. In a publication in the Journal of Physical Chemistry describing this same dealumination technique, the products are shown to have improved thermal stability as alumina is removed. This is further evidence that this method of dealumination does not produce a structure containing a large number of hydroxyl "nests" or defect sites. Hence, it is an example of Case 3a dealumination.